This invention relates to an austenitic, non-magnetic, stainless steel alloy and articles made therefrom and, more particularly, to such an alloy which, when significantly warm-worked but not subsequently annealed, has an outstanding combination of non-magnetic behavior, high yield strength, and good corrosion resistance, particularly resistance to chloride stress corrosion cracking.
Chromium-manganese stainless steel alloys are used in the manufacture of oilwell drilling equipment, including certain kinds of drill collars and housings for measurement-while-drilling (MWD) assemblies. More specifically, modern deep-well drilling methods, including directional drilling, require close monitoring of the location of the borehole to minimize deviations from the desired course. This may be accomplished by incorporating electrical measuring equipment in certain drill collar sections. However, since such measurements are disturbed by magnetic behavior, those drill collars containing such equipment must be non-magnetic, meaning here and throughout this application, having a relative magnetic permeability of less than about 1.02. Also, drill collars and other such articles are required to have high strength, particularly, a room temperature 0.2% offset yield strength of at least about 100 ksi. Chromium-manganese stainless steels have been favored in the manufacture of such articles because they satisfy both of these requirements at reasonable cost.
The following are hitherto known chromium-manganese stainless steel alloys, the compositions of which are listed in Table I: UNS S28200; UNS S21300; the experimental alloy described in V. Cihal and P. Pohoril, "Austenitic Chromium-Manganese Steels Resistant to SCC in Concentrated Chloride Solutions" in Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, 1170-1182, NACE (1977), identified here as Heat No. 7412; U.S. Pat. No. 3,075,839, issued to E. J. Dulis et al. on Jan. 29, 1963; U.S. Pat. No. 3,112,195, issued to H. Souresny on Nov. 26, 1963; U.S. Pat. No. 3,904,401, issued to D. L. Mertz et al. on Sept. 9, 1975 (UNS S28200 and UNS S21300 are both exemplary alloys of this patent); U.S. Pat. No. 4,514,236, issued to W. T. Cook et al. on Apr. 30, 1985; U.S. Pat. No. 4,523,951, issued to R. J. Andreini et al. on June 18, 1985; Duvall XM-19H; and U.S. Pat. No. 4,481,033, issued to K. Fujiwara et al. on Nov. 6, 1984. The foregoing alloys suffer from one or more deficiencies. For example, UNS S28201 and UNS S21300 (representative of the 3,904,401 patent) have less than desirable stress corrosion cracking (SCC) resistance. The alloy described by Cihal et al. contains excessive amounts of ferrite, causing undesirable magnetic behavior. Further, the balance of elements in these alloys reflects a lack of recognition of the important relationship between the manganese and the nickel and copper contents of the alloy on the one hand, and the chromium plus
TABLE I __________________________________________________________________________ w/o UNS UNS Cihal Duvall S28200 S21300 7412 '839 '195 '401 '236 '951 XM-19H '033 __________________________________________________________________________ C 0.15 max. 0.25 max. 0.05 0.15 max. 0.01-0.25 0.25 max. 0.50 max. 0.035 max. 0.046 0.05-0.18 Mn 17-19 15-18 15.49 11-14 12.00-25.00 15-20 10-25 16-20 18.57 16-25 Si 1 max. 1 max. 0.49 0-3 0.10-1.00 0-1 1 max. 0.73 0-1.0 Cr 17-19 16-21 19.22 14-18 10.00-20.00 16-22 0-20 12-15 11.98 14-17 Ni 3 max. 0.012 0.5 max. 5.00 max. 3 max. 1 max. 2.5 max. 0.31 0.06-0.3 Mo 0.5-1.5 0.5-3 0.524 0.3-3 1.00 max. 0.5-3 1.0 max. 5 max. 1.15 Cu 0.5-1.5 0.5-2 2 max. 0.5-2 0.03 N 0.4-0.6 0.2-0.8 0.364 0.15-0.55 0.05-0.50 0.2-0.8 0.20 min. 0.2-0.5 0.38 0.3-0.6 B 0.01 max. P 0.045 max. 0.05 max. 0.020 0.04 max. 0.05 max. 0.10 max. 0.022 S 0.030 max. 0.5 max. 0.009 0.04 max. 0.5 max. 0.10 max. Se 0.75 max. V 0.2-2.0 0.05 0.8 max. Nb 0.1 max. * Fe Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. __________________________________________________________________________ *w/o Nb .gtoreq. 10 w/o C
molybdenum contents on the other hand, in ensuring good resistance to SCC in chromium-manganese stainless steel alloys.
Recent developments in deep-well drilling methods have placed more stringent demands on parts such as drill collars. For instance, such parts are required to operate in increasingly severe chloride environments, for example, in contact with drilling muds containing high concentrations of chlorides, leading to increased risk of costly premature failure due to chloride stress corrosion cracking. Thus, a significant problem encountered by the oil drilling industry is that drill collars used to house critical measurement-while-drilling equipment, fabricated from known chromium-manganese stainless steel alloys, do not possess the requisite combination of non-magnetic behavior, high yield strength and good resistance to chloride stress corrosion cracking necessary for acceptable performance under more exacting operating conditions.